Shared Drafts for shared labels
This feature lets you share Gmail drafts with your team-mates with whom the email is shared with.
To use this, click on the button shown below while replying to an email
You’ll be able to save this as a Shared Draft in the dialog box that opens. The draft is automatically shared with everyone with whom the email is shared with. If you want you can also send a notification to one or multiple users about it.